Material removal process for self-aligned contacts

ABSTRACT

A method is disclosed of removing a first material disposed over a second material adjacent to a field effect transistor gate having a gate sidewall layer that comprises an etch-resistant material on a gate sidewall. The method includes subjecting the first material to a gas cluster ion beam etch process to remove first material adjacent to the gate, and detecting exposure of the second material during the gas cluster ion beam (GCIB) etch process.

DOMESTIC PRIORITY

This application is a Divisional of Non-Provisional application Ser. No.14/969,708, entitled “MATERIAL REMOVAL PROCESS FOR SELF-ALIGNEDCONTACTS,” filed Dec. 15, 2015 which is incorporated herein by referencein its entirety.

BACKGROUND

The present invention relates to field effect transistors (FET's), andmore specifically, to material removal techniques for formingself-aligned contact openings for field effect transistors.

Semiconductor devices such as FETs, MOSFET's (metal oxide semiconductorFET's), and FinFET's (fin-type FET's) are typically fabricated insequential processes involving steps materials of different types aredeposited or grown, and steps where materials of different types areremoved. For example, material is typically removed to form FET contactopenings. Various types of material removal processes have been used,such as chemical etching, plasma etching, or reactive ion etching (RIE).

In the past, removal of material for contact openings was dimensionallycontrolled through conventional masking techniques. With suchtechniques, a mask could be disposed over portions of the device wherematerial removal was not desired, for example by exposing and curing asoft photoresist mask and removing unexposed areas with solvent or byselectively depositing a hard mask material. A material removal processsuch as the above-described processes would then be applied through theopenings in the mask to remove the underlying material.

More recently, as semiconductor device density has increased, with aconcomitant decrease in component sizing and spacing, limitations werereached in the capability of conventional masking techniques to provideaccurate registration matching with smaller and more tightly spacedstructures beneath the mask. This led to the development ofself-aligning technologies such as self-aligned contact (SAC) etching.In a self-aligned contact etch process, adjacent gate structures areprovided with an etch-resistant material on the gate sidewall, whichprevents etching of the gate itself in the event of mis-alignment of themask opening edges with the interface between the material removal areaand the gate. In the SAC etch process, a material removal technique isused that is selective between the material to be removed and theetch-resistant material on the gate sidewall.

SUMMARY

According to an embodiment of the present invention, a method isprovided of removing a first material disposed over a second materialadjacent to a field effect transistor gate comprising a gate sidewalllayer that comprises an etch-resistant material on a gate sidewall. Themethod comprises subjecting the first material to a gas cluster ion beametch process to remove first material adjacent to the gate, anddetecting exposure of the second material during the gas cluster ionbeam (GCIB) etch process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D each sequentially depict an exemplaryembodiment of a FET gate structure subjected to a material removalprocess.

FIG. 2 schematically depicts an exemplary embodiment of a GCIB apparatusfor carrying out the above-described method.

FIG. 3 schematically depicts an exemplary embodiment of another GCIBapparatus for carrying out the above-described method.

FIG. 4 is a flowchart illustrating an example embodiment of a method ofGCIB etching of material adjacent to gate structures on semiconductorwafer.

FIGS. 5A and 5B each schematically depict an exemplary embodiment ofdeployment of a detector in an exemplary embodiment of theabove-described method.

DETAILED DESCRIPTION

With reference now to FIGS. 1A-1D, an exemplary embodiment of materialremoval adjacent an exemplary FET gate structure is schematicallydepicted. As shown in FIGS. 1A-1D, an intermediate semiconductor devicestructure 10 is ready for material removal for opening gate contacts. Inthe structure 10, gates 12 are disposed on substrate 14. Depending onthe application for which the FET is intended, the gates can havevarious different internal structures formed from various materials. Insome exemplary aspects, the gates 12 can be patterned from a stack ofmaterials including a thin layer of gate dielectric such as HfO₂, a gateconductor such as metal (e.g., aluminum or tungsten) or dopedpolysilicon on top of the gate dielectric, and optionally a gate hardmask such as silicon nitride on top of the gate conductor. The substrate14 can be formed from a variety of semiconductor materials, includingbut not limited to polysilicon, silicon-germanium, silicon carbide,indium, or gallium. In the exemplary embodiment of FIGS. 1A-1D, region16 of the substrate 14 is doped to provide electrical conductivity toserve as a source/drain region, although other source/drainconfigurations such as a raised source/drain can also be utilized. Itshould be noted that although FIG. 1A shows an already formedsource/drain region 16 as is commonly practiced, doping of the substrate14 can also be effected after formation of the contact opening beforesubsequent deposition of the contact material (not shown). Sidewallspacers 18 disposed on the sidewalls of the gates 12 can be formed froman etch-resistant material such as SiN, SiBCN, SiOCN, SiOC, and otherdielectrics formed from oxides and nitrides of silicon. and thus serveas the above-described etch-resistant material for the gate sidewalllayer. Alternatively, the sidewall spacers 18 can be formed from anon-etch resistant dielectric material such as silicon dioxide and becovered with a layer of etch-resistant material such as titaniumnitride. Although not required, in the exemplary embodiment shown inFIGS. 1A-1C, the etch resistant material is also disposed on thesubstrate 14 as etch stop layer 19 across the space between the gates12. As shown in FIGS. 1A and 1B, the gates 12 are surrounded by anetchable interlayer dielectric material 20 such as borophosphosilicateglass or phosphosilicate glass. Mask layer 22 can be a soft photoresistmask or a hard mask such as a hard oxide or nitride material.

In FIG. 1B, the mask layer 22 is patterned to provide an opening 24 overthe area from which material is to be removed. In the embodimentsdepicted in FIGS. 1A and 1B, the opening is formed lithographically in acontinuous soft mask layer by exposing a photoresist material to cure itin areas other than the mask opening 24 followed by solvent washing toremove uncured photoresist. Alternatively, in the case of a hard mask,the hard mask is deposited in the patterned configuration of FIG. 1B,bypassing the need for a configuration as shown in FIG. 1A. It should benoted that in FIG. 1B, the mask opening 24 is depicted as preciselyaligned with the space between the gates 12 from which material is to beremoved, but in many cases there would be some misalignment that wouldbe compensated for by self-aligned contact (SAC) techniques where anetch-resistant material on the gate sidewalls prevents unwanted etchingof either of the gates 12 in any area of overlap of the gate 12 and themask opening 24.

In FIG. 1C, etch processing is applied through the mask opening 24 asrepresented by arrow 26 to remove the interlayer dielectric material 20.This etch processing can be a gas cluster ion beam etch process, withdetection employed to identify exposure of the etch stop layer 19, orexposure of source/drain 16 if etch stop layer 19 is not present. Inembodiments where etch stop layer 19 is present, the interlayerdielectric material 20 can alternatively be removed down to the etchstop layer 19 by other etching processes such as chemical etching,reactive ion etching (RIE), or plasma etching (e.g., oxygen plasmaetching), and gas cluster ion beam etching can be used to remove etchstop layer 19 as shown in FIG. 1D. Although not required in allembodiments, GCIB etching can offer advantages in some embodiments byavoiding lag issues that can occur with other etching technologies suchas ME, and is also directional so that it can be used for contactopening in a wide variety of device configurations.

In a gas cluster ion beam process, a surface is bombarded by a beam ofhigh-energy gas phase atomic clusters. The clusters are formed when ahigh pressure gas (e.g., 10 atmospheres) is supersonically expanded intoa vacuum (e.g., 10⁻⁶ torr to 10⁻⁵ torr), where it cools and condensesinto weakly ionized clusters. The ionized clusters are electrostaticallyaccelerated to high velocities and focused into a beam that impacts thetarget surface. GCIB can be used for various applications such as filmdeposition, surface modification, or etching. Source gases for GCIBetching include various halogen gases, including but not limited to NF₃,SF₆, F₂, Cl₂, Br₂, and various halogen-substituted methanes substitutedwith 1 to 3 halogen atoms. Although not required in all embodiments,GCIB etching can offer advantages in some embodiments by avoiding lagissues that can occur with other etching technologies such as RIE, andis also directional so that it can be used for contact opening in a widevariety of device configurations.

As mentioned above, GCIB etching is used to remove a first materialadjacent to a FET gate while detecting for exposure of a second materialunder the first material. Although not required in all embodiments, insome embodiments such active detection can avoid over-etching intounderlying materials for which GCIB etch processing may lack selectivitysuch as for conductive materials or underlying doped or undopedsubstrate (e.g., Si, SiGe). Detection of exposure of an underlyingsecond material can be accomplished by optical sensing or by chemicalsensing. Optical sensing can be used when the underlying second materialhas different optical properties than the overlying first material.Chemical sensing can be performed by spectroscopy performed on gas inthe GCIB process chamber where the gas cluster ion beam contacts thetarget material. Examples of spectrometers that can be used for chemicalsensing include a quadrupole mass spectrometer residual gas analyzer(RGA) or a microplasma emission spectrometer (MPES).

An exemplary embodiment of a GCIB apparatus 100 is schematicallydepicted in FIG. 2. As shown in FIG. 2, a vacuum vessel 102 has acluster formation 104, an ionization/acceleration chamber 105, and aprocessing chamber 106. Although not required, a differential vacuumpumping system (not shown) can be utilized to maintain differentpressures in the different chambers (e.g., a pressure of 10⁻⁶ torr to10⁻⁵ torr in the ionization in the ionization/acceleration chamber 105and a pressure of 10⁻⁵ torr to 10⁻³ torr in the processing chamber 106).A source gas 112 is introduced through a gas feed tube 1914. Gasclusters 116 are formed by creating a supersonic jet of source gasthrough a nozzle 118 into the source chamber 104.

Cooling resulting from the expansion causes the gas to condense intoclusters of, for example, from several to several thousand atoms ormolecules. A gas skimmer aperture 120 defines a size of the beam of gasclusters 116 as it moves downstream toward target 121. After the clusterbeam has been formed, the clusters 116 are ionized in an ionizer 122.The ionizer 122 can be an electron impact ionizer that producesthermoelectrons from one or more incandescent filaments and acceleratesand directs the electrons causing them to collide with the gas clusters116 in the gas cluster beam where it passes through the ionizer 122. Theelectron impact ejects electrons from the clusters, causing the clustersto become positively ionized.

Accelerator 126 utilizes a series of electrodes to accelerate the beamto a desired energy, for example, from 1 keV to several tens of keV. Theaccelerated beam is directed at a substrate 128 for GCIB processing. Amass selector 128 can optionally be utilized for selecting clusters of acertain mass or within a certain range of masses. Such a mass selector128 can be, for example, a weak transverse magnetic field for deflectingmonomer ions and other light ions (e.g., those cluster ions of ten orfewer atoms or molecules) out of the beam and passing more massivecluster ions. A beam gate 130 is disposed along beam path, allowing thegas cluster ion beam to be interrupted in response to directions fromcontroller 136.

With continued reference to FIG. 2, detector 132 is connected to probe134 for detecting exposure of the second material underlying the firstmaterial as described above. As used herein, a “probe” can mean aconduit for sampling gas and transporting it to the detector 132 or anoptical probe for sensing an optical property of the etch target. Insome embodiments where detector 132 is a spectrometer such as aquadruple mass spectrometer residual gas analyzer (RGA), thespectrometer operates in a lower pressure range (e.g., 10⁻¹⁰ torr to5×10⁻⁴ torr) than the pressure in the process chamber 106 (e.g., 10⁻⁵torr to 10⁻³ torr). In such cases, the detector 132 can be maintained ata lower pressure than the process chamber by differential pumping withone or more orifices between the gas sampling location and the detector132 sized to maintain the pressure differential. FIG. 2 depicts anembodiment where gas can be collected or optical measurements taken by aprobe 134 located proximate to the intersection of the gas cluster ionbeam and the etch target 121 in the processing chamber 106. Such anembodiment allows for localized detection to be performed at the chipsite on a semiconductor wafer. In an alternative embodiment where suchlocalized detection is not needed or desired, detection can be performedat a non-localized (wafer) level, with a probe 134 located remote fromthe etch target 121 such as at or near the periphery of the processingchamber 106, as shown in FIG. 3.

Referring again to FIG. 2, controller 136 is operatively connected todetector 132 as represented by connection 138, to beam gate 130 asrepresented by connection 140, and to other system components asrepresented by connection 142. Any or all of these connections can bewired or wireless, and other connections can be established to any othersystem components not illustrated herein. Additionally, the controller136 can be a single controller or a plurality of interconnectedcontrollers. In some embodiments, the method further comprises, and thecontroller 136 can be configured to, terminate the gas cluster ion beametch process in response to detecting exposure of the second materialunder the first material. In some embodiments, the method furthercomprises, and the controller 136 can be configured to, determine anamount of time required for the gas cluster ion beam etch to remove thefirst material and expose the second material based on said detectingexposure of the second material.

In some embodiments, the methods described herein can be performed on asemiconductor device wafer that includes a plurality of gates. Althoughit is not required, in such embodiments an etch duration (i.e., anamount of time) can be determined for removal of the first material andexposure of the second material and etch-removal of first material canbe performed adjacent to one or more additional gates based on thedetermined etch duration without the need for active detection ofexposure of the second material. A non-limiting example embodiment of asuch a method is schematically depicted in FIG. 4. Referring now to FIG.4, from the start (block 402), the etch target 121 is subjected in block404 to GCIB etching for removal of first material. In some suchembodiments, the etch target 121 can be material adjacent to a gate 12(FIGS. 1A-1D) on a semiconductor device wafer and the etch duration canbe determined on a test chip on the wafer. Although it is not required,in some embodiments, the test chip can be located near the periphery ofthe wafer, as such chips are well-suited for test purposes as they areoften only partially printed and therefore not configured to beoperational. In some embodiments, the test chip area subjected to GCIBprocessing can be 1 cm² to 2 cm², and can include a number of gates. Inblock 406, exposure of a second material under the first material isperformed using techniques such as those described above. In block 410,another gate (e.g., on a production chip) in addition to the gate wheredetection was performed can then be etched based on the determined etchduration. A status check is performed in block 412 to determine whetherrecalibration is needed on another test chip, or if recalibration is notneeded, or if processing has been completed. If recalibration is needed,the process loops to block 404. The determination of whetherrecalibration is needed can be based on various protocols, for examplebased on a timer tracking cumulative GCIB etching time or a count timertracking a number of gates processed, or on a programming determinationthat all gates of a particular configuration on the wafer have beenprocessed. If recalibration is not needed, the process loops to block410 for GCIB etching on additional production chips, and the controller136 can be configured accordingly for carrying out such a method, withrepositioning of the gas cluster ion beam to different locationsaccomplished by repositioning target 121 with a positioning mechanism(not shown) in response to direction from the controller 136. In thisfashion, a number of additional gates on the wafer can be processed,with optional periodic etching of additional test chips during GCIB etchprocessing of the wafer, for example, to periodically check calibration.This approach can offer significant efficiency advantages by avoidingthe necessity of detecting exposure of the second material for each chipon the wafer. It should be noted however, that such advantages are notnecessary for all embodiments, and that some embodiments may not providethis advantage. If the status check in block 412 determines that allgates on the wafer designated for GCIB etching have been processed, theprocess ends at block 414.

In some embodiments, the method comprises positioning the probe 134, andthe controller 136 is configured to position the probe 134, proximate tothe point of intersection between the gas cluster ion beam and thetarget 121 during detection of exposure of the second material on a testgate or a test chip. The probe 134 is repositioned to a position remotefrom the point of intersection during gas cluster ion beam etching ofthe first material adjacent to additional gate or gates where etch isperformed for a determined duration based on detection of exposure ofthe second material on the test gate(s) or test chip(s). Positioning ofthe probe 134 can be effected by an electromechanical mechanism (notshown). Such an embodiment is depicted in FIGS. 5A and 5B, where FIG. 5Adepicts the probe 134 in a position proximate to from the point ofintersection during gas cluster ion beam etching for detection ofexposure of the second material on a test gate or a test chip, and FIG.5B depicts the probe 134 position remote from the point of intersectionduring gas cluster ion beam etching of the first material adjacent toadditional gate or gates where etch is performed for a determinedduration.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A gas cluster ion beam etching apparatus,comprising: a gas expander to form gas clusters; an ionizer and anaccelerator to form a gas cluster ion beam; a detector to detectexposure of a material at a target site during a gas cluster ion beametch process; and a controller to determine an amount of time requiredfor the gas cluster ion beam etch process to expose the material basedon detecting exposure of the material, wherein the detector includes aprobe movable between a position proximate to the target site and aposition remote from the target site, and the controller directs theprobe to the proximate position during etching of the target site anddirects the probe to the remote position during etching of one or moreadditional target sites.
 2. The apparatus of claim 1, wherein thecontroller terminates the gas cluster ion beam etch process in responseto detecting exposure of the material.
 3. The apparatus of claim 1,further comprising a spectrometer.
 4. The apparatus of claim 1, furthercomprising a quadrupole mass spectrometer residual gas analyzer.
 5. Theapparatus of claim 1, further comprising a microplasma emissionspectrometer.
 6. The apparatus of claim 1, further comprising an opticalprobe for sensing an optical property of the material at the targetsite.
 7. The apparatus of claim 1, further comprising a conduit forsampling gas and transporting it to the detector.
 8. A gas cluster ionbeam etching apparatus, comprising: a gas expander for forming gasclusters; an ionizer and an accelerator for forming a gas cluster ionbeam; a detector for detecting material at a target site of the gascluster ion beam; and a controller to identify exposure of a secondmaterial under a first material at the target site in response to inputfrom the detector, wherein the controller determines an etch durationneeded to expose the second material at the target site on asemiconductor device wafer, and subjects one or more additional targetsites on the wafer for said duration, and wherein the detector includesa probe movable between a position proximate to the target site and aposition remote from the target site, and the controller directs theprobe to the proximate position during etching of the target site anddirects the probe to the remote position during etching of said one ormore additional target sites.
 9. The apparatus of claim 8, wherein thecontroller terminates gas cluster ion beam etching in response toexposure of the second material.
 10. The apparatus of claim 8, whereinthe probe is an optical probe for sensing an optical property of theetch target.
 11. The apparatus of claim 8, wherein the probe is aconduit for sampling gas and transporting it to the detector.